Various charging systems for rechargeable batteries, such as nickel-cadmium (NiCd) batteries, are well known in the art. When a battery is deeply discharged, such a battery charger will typically provide a constant level of current until a battery property, such as cell voltage or temperature, reaches a cutoff level. Thereafter, several of the previously known chargers deliver constant current at a reduced magnitude, or intermittent trickle current to the battery, or a combination thereof.
Delivering constant current to a NiCd battery for an extended period of time causes excessive cell hating and gas buildup within the cell which weakens the electrolyte and greatly accelerates the decomposition of organic materials within the cells including plate separators and seals.
In commonly assigned U.S. Pat. No. 4,554,500 which issued Nov. 19, 1985, there is disclosed a battery charging method and apparatus which includes a programmable two-level constant current source. The battery is initially charged with a constant current for a predetermined time or until the battery temperature exceeds a threshold level, whichever comes first. If the temperature excess comes first, the battery charger then charges the battery at a reduced rate for an additional period of time to ensure that the battery is fully charged. Since cell temperature is a function of battery charge rate, the reduced charging rate minimizes overheating. After the predetermined or additional period of time elapses, as the case may be, the charger delivers pulsed current to prevent the battery from self-discharging.
Other presently known charging systems deliver a constant current to the battery until the battery exhibits a threshold voltage.
In order to charge a plurality of batteries, some charging systems of the prior art are configured in parallel, whereby each battery is charged independently of all others. Not every battery (or "battery pack") in a group of batteries to be charged will have the same level of discharge. Therefore, in parallel battery charging systems of the prior art, if the system has a moderate, total current output and provides an equal fraction of the total current to each battery, the time required to fully charge one battery is relatively long compared to sequential charging systems.
Other battery chargers of the prior art individually charge each battery to full charge in a sequential fashion. In sequential charging systems, no distinction is made between batteries that are only slightly discharged and those which are more fully discharged in determining the order of charging, and each battery is charged to its fully charged state with a constant current. Moreover, some battery chargers of the prior art cannot determine the type of battery to be charged, (e.g., 2 or 4 ampere-hours; 12, 13 or 14 volt), and may require that the batteries be segregated according to type in order to optimally charge them.
An object of the present invention is to provide a battery charging system that will prioritize a plurality of batteries and charge the batteries in the most time efficient manner for the group, e.g., wherein those batteries needing the least charge are charged first.
Another object of the present invention is to provide a battery charging system that automatically adjusts charging parameters in accordance with battery type.
Still another object of the present invention is to provide a battery charging system which will provide pulsed current to the batteries when appropriate to avoid overheating and gas buildup in the battery cells.